1. Field of the Invention
The present invention relates to a liquid discharging method and a liquid discharging apparatus and particularly those using a liquid discharging head including a discharge port for discharging a liquid and an individual liquid chamber which communicates with the discharge port and discharges the liquid from the discharge port according to volume variation. The invention is applicable to an apparatus for performing a recording operation on a recording medium such as a paper sheet, cloth, leather, non-woven, or an OHP plastic film, an apparatus for performing a patterning or processing operation on a medium (receptor) such as a substrate, plate material, or a solid material by attaching the liquid thereto, and a coating apparatus.
2. Description of the Related Art
There is a known liquid discharging head in which a diaphragm contacting a liquid in an individual liquid chamber is provided, the diaphragm is displaced by a piezoelectric film which is deformed according to application of a voltage, and then the volume of the individual liquid chamber is increased or decreased so as to eject an ink by a change of pressure caused thereby.
In such a liquid discharging head having the above-described configuration, the discharged liquid extends in a column shape and separates halfway. Accordingly, a separated liquid droplet reaches a liquid receptor such as a recording medium. At this time, a subsidiary liquid droplet called a satellite may be generated, in addition to the intended liquid droplet (main droplet) to reach the receptor. Generally, the satellite is smaller than the main droplet, and the speed of the satellite is slower than that of the main droplet. Accordingly, the satellite may be deposited at a position deviated from the main droplet in the liquid receptor such as the recording medium, which may cause deterioration of recording quality and patterning precision. Additionally, the satellite may float in the form of a mist, and may be attached to an discharge port formation surface of the liquid discharging head, by which a direction of the liquid to be subsequently discharged may be diffracted or attached to the inside of the apparatus, which may cause contamination of the apparatus.
In order to solve such a problem, in the past, there have been proposed various technologies of preventing the satellite from being generated.
For example, Japanese Patent Application Laid-Open No. H05-057888 (EP A1 0531173) discloses a method for restricting the satellite from being generated in the configuration including a piezoelectric element which is deformed according to application of a voltage so as to increase or decrease the volume of the individual liquid chamber (ink chamber). In such a method, in order to decrease the volume of the individual liquid chamber, a braking pulse is instantaneously applied to the piezoelectric element after a driving voltage to the piezoelectric element has reached a peak level so that inertia of both the piezoelectric element and the ink contained in the ink chamber is immediately canceled to abruptly stop the ink discharge. A driving voltage waveform shown in FIG. 1A of Japanese Patent Application Laid-Open No. H05-057888 (EP A1 0531173) has an abrupt region.
Additionally, a method of driving a piezoelectric vibrator is disclosed in Japanese Patent Application Laid-Open No. H07-076087 (U.S. Pat. No. 5,453,767). In such a method, the piezoelectric vibrator is driven so that the volume of the individual liquid chamber is increased, the volume is decreased at a first changing speed, and the volume is decreased at a second changing speed faster than the first changing speed. Accordingly, a speed difference between the front end and the rear end of the liquid column (ink column) is decreased to thereby form a spherical ink droplet.
In the method disclosed in Japanese Patent Application Laid-Open No. H05-057888 (EP A1 0531173), the braking pulse is instantaneously applied to the piezoelectric element after the driving voltage to the piezoelectric element has reached the peak level. As shown in FIG. 1A, it is appropriate to consider that the braking pulse is instantaneously applied to the piezoelectric element at the time of reaching the peak level. However, there is no detailed description about how the peak level is set and how the abrupt gradient of each region of the driving voltage waveform is estimated. Additionally, the ink discharging operation at the time of application of the driving voltage having such a waveform is not described in detail with reference to the drawings. Specifically, it is not clear how the satellite is prevented from being generated.
Meanwhile, in the method disclosed in Japanese Patent Application Laid-Open No. H07-076087 (U.S. Pat. No. 5,453,767), the speed difference between the front end and the rear end of the ink droplet is decreased compared with a conventional example, but the speed of the front end is still faster than that of the rear end. Accordingly, it can be easily expected that a plurality of satellites are generated based on the ratio between the thickness and the length of the liquid column shown in FIG. 7VIII of Japanese Patent Application Laid-Open No. H07-076087 (U.S. Pat. No. 5,453,767).